Electrowetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al, issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets and the mixing together of droplets of different materials. In general the voltages required to perform typical droplet operations are relatively high. Values in the range 20-60V are quoted in the prior art (e.g. U.S. Pat. No. 7,329,545 (Pamula et al., issued Feb. 12, 2008)). The value required depends principally on the technology used to create the insulator and hydrophobic layers.
A notable feature of the EWOD actuation mechanism is that the contact angle of the liquid droplet with the solid surface depends on the square of the actuation voltage; the sign of the applied voltage is unimportant to first order. It is thus possible to implement EWOD with either an AC or a DC drive scheme. FIG. 1 shows a typical timing sequence for an AC drive scheme. A voltage VT is applied to the electrode of the top substrate and for the simplicity of what follows may be assumed to be ground. In the case where the EW drive electrode is programmed low, the voltage VT is also applied to the EW drive electrode. In the case where the EWOD drive electrode is programmed high, a voltage V1 is applied to the EW drive electrode. V1 is a square waveform of amplitude 2VA, high level +VA and low level −VA. In the case where the frequency of the AC waveform is below the characteristic droplet response frequencies (as determined by the droplet conductivity), the electrowetting voltage VEW is given by the root mean square (rms) value of the voltage difference between V1 and VT, equal to VA.
There are several advantages of implementing EWOD with an AC drive scheme. These advantages include:                Reduced device degradation through life        Improved insulator reliability        Improved droplet dynamicsFor these reasons most groups working on EWOD use AC drive schemes with drive frequencies of typically 10 kHz or higher.        
Many modern liquid crystal (LC) displays use an Active Matrix (AM) arrangement whereby thin-film transistors control the voltage maintained across the liquid crystal layer.
LC displays generally require that the voltage across the liquid crystal should be alternated (“inversion”) since the application of a DC field has deleterious effects for the LC material. Most LC inversion schemes operate so as to invert the sign of the applied LC voltage with each frame of information written to the display. This is typically a frequency of 50-60 Hz.
“Ultra-Low Power System-LCD with Pixel Memory Circuit”, Matsuda et al., Proceedings of IDW '09, AMD1-2, describes an LCD with a pixel memory driving scheme. Pixel memory refers to a technology whereby the data written to the display is held by an SRAM memory cell within the pixel. The display is thus 1-bit, i.e. it can only display black or white and not intermediate grey levels. The advantage of such an implementation is that it removes the requirement to periodically refresh the voltage written to the display and thus reduces power consumption. In order to effect inversion of the voltage across the LC layer, an additional inversion circuit is also included in pixel. This enables the inversion frequency to be higher than the data refresh rate of the display.
U.S. Pat. No. 7,163,612 (J. Sterling et al.; issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
Such an approach may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based electronics to control an EWOD array, namely:                Driver circuits can be integrated onto the AM-EWOD array substrate.        TFT-based electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
A disadvantage of U.S. Pat. No. 7,163,612 (J. Sterling et al.; issued Jan. 16, 2007) is that it does not disclose any circuit embodiments for realising the TFT backplane of the AM-EWOD. This patent also specifies use of a ground plane on the top substrate. This has the disadvantage that the voltages switched by the TFTs must be at least equal to the electrowetting drive voltage.
In some cases however, the EWOD actuation voltage may still exceed the maximum voltage rating of the TFTs. High voltage operation of TFTs may result in device degradation or failure as is well known.
A further disadvantage of the high operating voltage is that power consumption from operating the switches (due to the charging and discharging of the parasitic switch capacitance) may also be significant. This power consumption scales with the square of the voltage amplitude.
An alternative technology for implementing droplet microfluidics is dielectrophoresis. Dielectrophoresis is a phenomenon whereby a force may be exerted on a dielectric particle by subjecting it to a varying electric field. “Integrated circuit/microfluidic chip to programmably trap and move cells and droplets with dielectrophoresis”, Thomas P Hunt et al, Lab Chip, 2008, 8, 81-87 describes a silicon integrated circuit (IC) backplane to drive a dielectrophoresis array for digital microfluidics. This reference describes an integrated circuit for driving AC waveforms to array elements, shown FIG. 2. The circuit consists of a standard SRAM memory cell 104 to which data can be written and stored, switch circuitry 106, and an output buffer stage 108. According to the operation of the switches 110 and 112 either the AC signal Vpix (shown as a 5V 1 MHz square wave in this example) or complementary signal Vpix is written to the pixel. Unlike in the case of an EWOD array, the dielectrophoresis system does not require a top substrate. Furthermore, unlike EWOD such a dielectrophoresis system can operate with relatively low drive voltages, typically 5V. This is compatible with typical operating voltages for silicon ICs and TFTs used in displays.